This invention relates to an improvement in the processing of filled thermoplastic resins into shaped articles by the addition of a dispersion aid in the form of small amounts of fibrous materials having an aspect ratio at least about 10 times that of the filler.
The addition to molding compositions of quantities of up to about 80% particulate fillers is known in the thermoplastic art. The use of these fillers offers many advantages such as improved stiffness, thermal and dimensional stability, or improved mechanical properties, and includes as well the benefit from diluting the polymer to produce a more economical product. However, when amounts of filler in excess of about 15% are employed, difficulties in processing techniques arise. According to the amount of filler present, it becomes increasingly difficult to thoroughly disperse the filler as well as to feed or extrude it, and excessively high power requirements are often necessary to effect processing. In many applications where relatively high amounts of filler are used, it may be necessary to process mixtures more than once to obtain suitable homogeneity, e.g., in order to produce a mixture containing greater than about 50% filler using conventional techniques, it is often necessary to first combine all the resin with half the filler, extrude or otherwise process this mixture and then add the remaining filler and reprocess the mixture to effect proper dispersion. Not only is this process awkward and time consuming, but it requires a large amount of extra mixing equipment.
Alternatively, expensive processing equipment such as continuous Banbury mixers, etc., may be used. The use of screw extruders with very high compression ratios to blend highly filled mixtures, causes significant wear on the extruder and the products often contain deleterious amounts of material from the screw and barrel. During processing, undispersed material may accumulate in dead spots in the equipment, e.g., behind the breaker plates, etc. The undispersed particles may also accumulate in the screen area of the equipment causing clogging of the screens which will slow down or completely stop processing operations and may result in a poorer quality product. In some cases, water is added to help densify and disperse the mixtures, but this procedure is not always effective and it is necessary to remove the water after dispersion has been completed; in addition, some polymers are sensitive to moisture e.g., polyamide and polyesters, and can hydrolytically degrade in the presence of moisture at elevated temperatures.
It is also difficult to feed such highly filled thermoplastic compositions in the hopper and much physical manipulation is usually required to prevent bridging of the feed stock. The uneven feeding in the hopper also leads to uneven filling of the screw flights thereby causing surging at the outlet of the extruder.
Moreover, because of the difficulty of obtaining good mixing, the quality, uniformity and homogeneity of the product resulting from compounding of these highly filled systems is often very poor. Thus, the products formed therefrom often exhibit poor surface and mechanical properties characterized in part by agglomerated clumps of filler and the presence of voids.
The use of long glass fibers in quantities of about 10% or greater is known to reinforce filled and/or unfilled thermoplastic compositions as described in U.S. Pat. Nos. 3,419,517, 3,639,331 and 3,503,919 as well as in our copending application Ser. No. 327,284, filed Jan. 29, 1973. The reinforced thermoplastic thus produced using long glass fibers are usually in sheet form and are adapted for use only in molding apparatus since the reinforcing advantages of these long fibers are lost during any melt processing techniques due to fiber breakup. It is also possible to use short glass fibers as fillers but when used in large amounts, the economical advantages inherent in the use of fillers as dilutants are lost.
The addition of small amounts of fibrous reinforcement is also known in the art, however, it has been found that such amounts often lower the mechanical strength of the composites. This is because fibrous reinforcement must be present in at least a certain critical minimum amount to be able to function as a load-bearing agent and at concentrations below this amount, fibers often stiffen the matrix without strengthening it. The "critical" or minimum loading level below which no strengthening is achieved is on the order of about 6 to 15% by total weight of the composites.
There is thus a need for an economical improved filled thermoplastic composition adaptable to easier melt processing and which will provide a filled thermoplastic product which can be extruded to produce articles possessing superior homogeneity, quality and uniformity.
There is also a need for a method for thoroughly dispersing large quantities of filler in thermoplastic compositions in a one-step processing operation.